Figure 8.

Theta nonlinearity of the three rats tested in the dorsal (left) and intermediate (right) CA1 regions and low (green) and high (purple) running velocities. The nonlinearity measure ϕ (Eq. 12) is defined only for bispectral values exceeding the 95% confidence level of the zero modulus of the normalized bispectrum (Eq. 13): the missing low-speed bar for rat 7805 for intermediate CA1 indicates that, at low speed, the normalized bispectrum is statistically zero. There was a significant main effect of hippocampal region (F_{(1, 4)} = 30.6, p < 0.01; repeated measures) on the magnitude of the nonlinearity such that the EEG in the dorsal hippocampus was more nonlinear than the intermediate. There was not a significant main effect of velocity (low vs high) on nonlinearity (F_{(1, 4)} = 5.9, p = 0.08; repeated measures), but the interaction between hippocampal region and velocity was significant (F_{(1, 4)} = 9.1, p < 0.05; repeated measures). Post hoc analysis indicated that this interaction effect was due to a significant difference in EEG nonlinearity across velocity conditions in the dorsal hippocampus (p < 0.05), but not in the intermediate hippocampus (p = 0.2).